The published German Patent Application DE 10 2012 209883 A1 discloses a lever system for operating a torque transmission device such as a clutch in a drive train of a motor vehicle. The lever system comprises a lever supported in a bearing location to rotate about a joint and having a force introduction location that is movable in parallel with the axis of rotation of the torque transmission device. The force introduction location is in effective connection with an operating actuator. The actuator itself may be any type of linear actuator, for instance a planetary roller spindle drive actuator (PWG), a hydraulic slave cylinder, etc. Planetary roller spindle drives (PWG) are formed of a spindle, a spindle nut and planetary rolling bodies disposed there between along the circumference in a planet carrier. The spindle, spindle nut and planetary rolling bodies have profiles to transmit a rotary movement between the spindle and the spindle nut. One of the components, for example the spindle or spindle nut, is driven to rotate and the other component is fixed along the longitudinal axis of the spindle against relative rotation and is moved through an axial path as a function of the set transmission ratio. The planetary rolling bodies have two different profile sections, meshing with the spindle on the one hand and with the spindle nut on the other hand. The one profile sections on the planetary rolling body are groove-shaped and the other profile sections on the planetary rolling body are generally groove-shaped and, depending on the embodiment, a complementary thread section is provided on the spindle or on the spindle nut and the groove section is provided on the other component in a corresponding way. Compared to a direct arrangement of the spindle on the spindle nut, this arrangement provides transmission ratios that allow a higher force for every revolution of the spindle with a shorter feed motion.
EP 320 621 A1 for instance discloses a PWG wherein the spindle has a thread and the spindle nut has grooves on which respective planetary rolling bodies roll off by means of corresponding profiled sections of complementary design. The spindle, which is axially fixed and driven to rotate by an electric motor, for instance, is rotated relative to the spindle nut, which is fixed against relative rotation, in a way to force the spindle nut to be axially displaced due to the frictional rolling of the planetary bodies in the grooves of the spindle nut. Via the axial displacement of the spindle nut, this results in a movement that is proportional to the rotary speed of the spindle. For this purpose, in a manner corresponding to planetary gears, the planet carrier with the planetary rollers rotates at half the rotary speed or at a different ratio, depending on the diameters of the rotating component, namely the spindle or spindle nut.
DE 10 2010 047 800 A1 discloses an application of a planetary gear set wherein an actuator part of a hydrostatic actuator, the actuator part embodied as a piston of a master cylinder, is coupled in an axially fixed way with the spindle, which is disposed to be fixed against relative rotation and axially movable. The axially fixed spindle nut is driven to rotate by an electric motor so that when the spindle nut is rotated, the spindle and spindle nut are displaced relative to one another along the axial path. As the cylinder housing receiving the piston and forming a pressure chamber with the latter is fixed and the spindle nut is axially supported thereon, depending on the rotary driving of the spindle nut, the piston builds up a pressure for instance for actuating motor vehicle components such as brakes, friction clutches and the like. In this context, it is desirable especially for friction clutches to quickly overrun a clearance with low load requirement by means of a high pitch and in the region of operation of the friction clutch to be able to apply high loads using a low-power electric motor by means of a small pitch.
In particular in actuators such as hydrostatic clutch actuators (HCA) that operate against a load, for instance a clutch characteristic curve, a problem of spindle drives (e.g. PWGs) that are driven by an electric motor, for instance, and are optimized in terms of their degree of efficiency, is that when a position needs to be held, a holding current and thus a holding torque is required in the electric motor because the spindle drive (e.g. in a PWG) is not self-locking. In a PWG in a HCA with a ring gear nut that is fixed against relative rotation and a driven spindle from which the axial movement (relative to the spindle) is taken, in the case of an ideal rolling movement (like in a splined planetary gear/no slip), an additional transmission of the spindle pitch occurs, which is freely selectable within limits by varying the spindle and/or planetary roller diameters.
A decisive disadvantage of actuators of this type is that they require a continuous holding current and that the position cannot be held in the case of a power failure/malfunction (cable break/plug power dip). A clutch that is pushed open by the actuator then risks closing unintentionally.
In addition, it is known to use of wrap springs, for instance, in clutches. The wrap spring clutch consists of a coil spring that is wound onto a shaft or a cylindrical body and is fixed to the drive on one side. The entrainment effect is based on the fact that the entrainment moment increases and adds up with each revolution due to friction. As a result, the force creating the friction increases at the same time due to the wrap-around arrangement. Only little friction occurs in the reverse direction—the diameter of the spring increases slightly, but the spring is not unwound. The friction that remains in a freewheel operation is frequently used as an overload clutch (c.f., Wikipedia).
The published German Patent Application No. DE 10 2007 047 394 A1, for instance, discloses a torsional vibration damper comprising at least two parts that are rotatable relative to one another against the effect of a damping device with energy storage elements effective in a circumferential direction, with one part connectible to a drive motor and the other to a shaft to be driven, wherein a torque limiter in serial arrangement with the damping device is additionally provided between the two parts, said torque limiter effective in at least one of the directions of relative rotation possible between the two parts and comprising at least one wrap spring clutch that acts as a freewheel when a pre-defined angle of rotation between the two parts is reached and comprises at least one wrap spring element. This solution is characterized in that at least one end of the wrap spring element is connected to a ring element so as to be fixed against relative rotation, the ring element having support regions for the energy storage elements effective in a circumferential direction and/or the ring element having actuation regions for the freewheel effect.